A problem with flying spot telecines and cathode ray tubes in general is burn. This is caused by the flying spot moving too slowly or stopping altogether at a point on the CRT face. The result is that the phosphor on the screen at this point is burnt which seriously degrades the response of that part of the screen.
We have appreciated that there are a number of types of fault or operating conditions which could lead to phosphur burn and which it would be desirable to protect against in high power cathode ray tubes such as telecine flying spot scanners.
A first situation occurs where there is a high concentration of beam energy on a single spot or scan location for a very short time; that is a few microseconds. A second situation is where there is a high concentration of beam energy in certain areas over longer periods of time; that is, a few milliseconds.